Predefined files upload to web application (by HTML or different solution)

On our company we have the solution inside web application where customer on web application can pre-define paths to files on disk (i.e: c:/invoices/summary.txt).

Afterwards by Java Applet we are reading files under defined paths to web application and modify the files on user disk.

Yes - here the problem appears - the Java Applets. The Chrome does not support it anymore and it is a legacy / unsecured solution. We are looking the replacement for this solution which allows us to modify the files on user disk and will support different Browsers / OS.

We considered the following solutions:

• dedicated external apps for OS which will be upload the defined files to our system (like Dropbox) but the problem is that we have to develop application for various OS and support it - a lot of work
• WebDav API and solution where customer download the script from web app and run it, script upload the files to server - the problem is that the script could be error prone
• HTML 5 solutions - we considered it but we want to achieve the situation that user defines the path only once

Does anyone see other sensible solutions for this case worth to try?